In view of decreasing harmful effects on the environment, in recent years, extensive research and development work has been undertaken to provide a hybrid vehicle wherein an engine and a motor are mounted as power sources to drive the vehicle.
Of these hybrid vehicles, a so-called parallel hybrid vehicle is known. The parallel hybrid vehicle is a type of hybrid vehicle having an engine and a motor arranged in parallel, and in accordance with a required driving force or road conditions, at least one of the engine and the motor is selectively driven or both of the engine and the motor are driven. In the parallel hybrid vehicle, an operating ratio of the engine and the motor can be selected in accordance with the required driving force or road conditions, which leads to improvement of the fuel economy and decrease in the amount of exhaust gas. The parallel hybrid vehicle further improves fuel economy by regenerative braking during deceleration of the vehicle. During regenerative braking, the motor converts kinetic energy of the vehicle into electric energy to charge the battery.
As a conventional parallel hybrid vehicle to perform regenerative braking (hereinafter simply referred to as a “hybrid vehicle”), Japanese Laid-open patent application Ser. No. 9-9415 discloses a hybrid vehicle where in the transmission sets in a higher speed stage during the deceleration of the vehicle. For example, when the driver applies a brake while a hybrid vehicle with a four speed automatic transmission runs at the third speed, the transmission is set in the fourth speed, and with the transmission kept in the fourth speed stage, the vehicle decreases and stops by regenerative braking. Because the drag resistance of the engine can be decreased with the transmission set in a higher speed stage during the deceleration of the vehicle, it is possible to decrease heat energy to be consumed as friction heat caused by the engine brake. This leads to increased regenerative electric energy and thus increased amount of regenerative electric power to be charged in the battery that is connected to the motor.
However, if this hybrid vehicle reaccelerates at the time of deceleration, a difference may arise between rotation speeds of the wheels, engine speed and the current speed stage of the transmission. Such a difference unpreferably causes a shock when the transmission sets in a speed stage.
In the case where the amount of regenerative electric power is increased by regenerative braking, it is preferable that the drag resistance of the engine is decreased as much as possible. However, in order to decrease the drag resistance of the engine further, it is necessary to provide a further transmission speed stage corresponding to a higher speed stage or to provide a mechanism which engages or disengages driving systems of the engine and the motor, which requires a novel construction as well as complicated controls.